Au(I)/Au(III) Catalytic Allylation Involving π-Allyl Au(III) Complexes

π-Bonding of Group 11 Metals to a Tantalum Alkylidyne Alkyl Complex Promotes Unusual Tautomerism to Bis-alkylidene and CO2 to Ketenyl Transformation

A salt metathesis synthetic strategy is used to access rare tantalum/coinage metal (Cu, Ag, Au) heterobimetallic complexes. Specifically, complex [Li(THF)2][Ta(CtBu)(CH2tBu)3], 1, reacts with (IPr)MCl (M = Cu, Ag, Au, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to afford the alkylidyne-bridged species [Ta(CH2tBu)3(μ-CtBu)M(IPr)] 2-M. Interestingly, π-bonding of group 11 metals to the Ta─C moiety promotes a rare alkylidyne alkyl to bis-alkylidene tautomerism, in which compounds 2-M are in equilibrium with [Ta(CHtBu)(CH2tBu)2(μ-CHtBu)M(IPr)] 3-M. This equilibrium was studied in detail using NMR spectroscopy and computational studies. This reveals that the equilibrium position is strongly dependent on the nature of the coinage metal going down the group 11 triad, thus offering a new valuable avenue for controlling this phenomenon. Furthermore, we show that these uncommon bimetallic couples could open attractive opportunities for synergistic reactivity. We notably report an uncommon deoxygenative carbyne transfer to CO2 resulting in rare examples of coinage metal ketenyl species, (tBuCCO)M(IPr), 4-M (M = Cu, Ag, Au). In the case of the Ta/Li analogue 1, the bis(alkylidene) tautomer is not detected, and the reaction with CO2 does not cleanly yield ketenyl species, which highlights the pivotal role played by the coinage metal partner in controlling these unconventional reactions.

Gold-Catalyzed C-N Cross-Coupling Reactions of Aryl Iodides with Alkyl Nitriles or Silver Cyanate

We have gold-catalyzed C-N cross-couplings of aryl iodides with aliphatic nitriles. Although nitriles are usually challenging nitrogen cross-coupling partners, they could be activated by base-mediated deprotonation and isomerization. The method utilizes widely available substrates in moderate to good yields to provide various N-aryl compounds. In addition, a similar strategy could be extended to the cross-couplings of aryl iodides with silver cyanate. The protocol features high humidity/air tolerance and works inter- and intramolecularly.

Consolidation of the Oxidant-Free Au(I)/Au(III) Catalysis Enabled by the Hemilabile Ligand Strategy

In this minireview we survey the challenges and strategies in gold redox catalysis. Gold's reluctance to oxidative addition reactions due to its high redox potential limits its applicability. Initial attempts to overcome this problem focused on the use of sacrificial external oxidants in stoichiometric amounts to bring Au(I) compounds to Au(III) reactive species. Recently, innovative approaches focused on employing hemilabile ligands, which are capable of coordinating to Au(I) and stabilizing square-planar Au(III) intermediates, thus facilitating oxidative addition steps and enabling oxidant-free catalysis. Notable examples include the use of the (P^N) bidendate MeDalphos ligand to achieve various cross-coupling reactions via oxidative addition Au(I)/Au(III). Importantly, hemilabile ligand-enabled catalysis allows merging oxidative addition with π-activation, such as oxy- and aminoarylation of alkenols and alkenamines using organohalides, expanding gold's versatility in C-C and C-heteroatom bond formations and unprecedented cyclizations. Moreover, recent advancements in enantioselective catalysis using chiral hemilabile (P^N) ligands are also surveyed. Strikingly, versatile bidentate (C^N) hemilabile ligands as competitors of MeDalphos have appeared recently, by designing scaffolds where phosphine groups are substituted by N-heterocyclic or mesoionic carbenes. Overall, these approaches highlight the evolving landscape of gold redox catalysis and its tremendous potential in a broad scope of transformations.

Gold-Catalyzed Arylative Cope Rearrangement

Cope rearrangements have garnered significant attention owing to their ability to undergo structural reorganization in stereoselective manner. While substantial advances have been achieved over decades, these rearrangements remained applicable exclusively to parent 1,5-hexadienes. Herein, we disclose the gold-catalyzed arylative Cope rearrangement of 1,6-heptadienes via a cyclization-induced [3,3]-rearrangement employing ligand-enabled gold redox catalysis. Detailed mechanistic investigations including several control experiments, cross-over experiment, HRMS analysis, 31P NMR and DFT studies have been performed to underpin the mechanism.

(P^N^C) Ligands to Stabilize Gold(III): A Straightforward Access to Hydroxo, Formate, and Hydride Complexes

A novel class of (P^N^C) pincer ligands capable of stabilizing elusive gold(III) species is reported here. Straightforward access to (P^N^C)gold(III) hydroxo, formate, and hydride complexes has been streamlined by first incorporating a cycloauration step devoid of toxic metals or harsh conditions. The resulting gold complexes exhibit remarkable stability in solution as well as in the solid state under ambient conditions, which enabled their characterization by X-ray diffraction analyses. Interestingly, the influence of the ligand allowed the preparation of gold(III)-hydrides using mild hydride donors such as H-Bpin, which contrasts with sensitive super hydrides or strong acids and cryogenic conditions employed in previous protocols. A detailed bonding characterization of these species is complemented by reactivity studies.

Ligand-Enabled Oxidative Fluorination of Gold(I) and Light-Induced Aryl-F Coupling at Gold(III)

MeDalphos Au(I) complexes featuring aryl, alkynyl, and alkyl groups readily react with electrophilic fluorinating reagents such as N-fluorobenzenesulfonimide and Selectfluor. The ensuing [(MeDalphos)Au(R)F]+ complexes have been isolated and characterized by multinuclear NMR spectroscopy as well as X-ray diffraction. They adopt a square-planar contra-thermodynamic structure, with F trans to N. DFT/IBO calculations show that the N lone pair of MeDalphos assists and directs the transfer of F+ to gold. The [(MeDalphos)Au(Ar)F]+ (Ar = Mes, 2,6-F2Ph) complexes smoothly engage in C-C cross-coupling with PhCCSiMe3 and Me3SiCN, providing direct evidence for the oxidative fluorination/transmetalation/reductive elimination sequence proposed for F+-promoted gold-catalyzed transformations. Moreover, direct reductive elimination to forge a C-F bond at Au(III) was explored and substantiated. Thermal means proved unsuccessful, leading mostly to decomposition, but irradiation with UV-visible light enabled efficient promotion of aryl-F coupling (up to 90% yield). The light-induced reductive elimination proceeds under mild conditions; it works even with the electron-deprived 2,6-difluorophenyl group, and it is not limited to the contra-thermodynamic form of the aryl Au(III) fluoride complexes.

Syn-Aminoauration versus Anti-Aminoauration of Alkynes in Au(I)/Au(III) Catalysis: Understanding the Origin of Selectivity

There is no experimental evidence of whether such gold-catalyzed aminoauration reactions follow the anti- and/or syn-pathway, and hence, to understand the origin of the selectivity in Au(I)- and Au(III)-catalyzed reactions of alkynes, a thorough mechanistic study was performed using DFT methods. The NBO and ASM analyses provided significant information about the structure-stability-reactivity of the pathway-determining states (PDS). This study further reveals that the oxidation states and geometries of gold, the steric bulk, and the dihedral angles of the PDS direct the mechanistic pathways and control the turnover frequency.

DFT-Enabled Development of Hemilabile (P∧N) Ligands for Gold(I/III) RedOx Catalysis: Application to the Thiotosylation of Aryl Iodides

Ligand-enabled oxidative addition of Csp2-X bonds to Au(I) centers has recently appeared as a valuable strategy for the development of catalytic RedOx processes. Several cross-coupling reactions that were previously considered difficult to achieve were reported lately, thus expanding the synthetic potential of gold(I) complexes beyond the traditional nucleophilic functionalization of π-systems. MeDalPhos has played an important role in this development and, despite several studies on alternative structures, remains, so far, the only general ligand for such process. We report herein the discovery and DFT-enabled structural optimization of a new family of hemilabile (P∧N) ligands that can promote the oxidative addition of aryl iodides to gold(I). These flexible ligands, which possess a common 2-methylamino heteroaromatic N-donor motif, are structurally and electronically tunable, beyond being easily accessible and affordable. The corresponding Au(I) complexes were shown to outperform the reactivity of (MeDalPhos)Au(I) in a series of alkoxy- and amidoarylations of alkenes. Their synthetic potential and comparatively higher reactivity were further highlighted in the thiotosylation of aryl iodides, a challenging unreported C-S cross-coupling reaction that could not be achieved under classical Pd(0/II) catalysis and that allows for general and divergent access to aryl sulfur derivatives.

Iodine-dependent oxidative regioselective aminochalcogenation of indolines

A directing-group-free strategy for oxidative regioselective aminochalcogenation of indolines with amines and dichalconides is presented. This strategy combines tandem coupling sequences and oxidative dehydrogenation methods in a multi-component reaction, enabling the fast construction of a series of C2,3- or C2,5-aminochalcogenated indole derivatives. Moreover, the application of this synthetic approach is demonstrated through the late-stage modification of pharmaceuticals and the derivatization of the products, highlighting its potential and significance.

Cyclometalated (N,C) Au(III) Complexes: The Impact of Trans Effects on Their Synthesis, Structure, and Reactivity

ConspectusThe early years of gold catalysis were dominated by Au(I) complexes and inorganic Au(III) salts. Thanks to the development of chelating ligands, more sophisticated Au(III) complexes can now be easily prepared and handled. The choice of the ancillary ligand has great consequences for the synthesis, properties, and reactivity of the Au(III) complex in question. Among the major factors controlling reactivity are the "trans effect" and the "trans influence" that a ligand imparts at the ligand trans to itself. The kinetic trans effect manifests itself with an increased labilization of the ligand trans to a given ligand and arises from an interplay between ground-state and transition-state effects. The term trans influence, on the other hand, is a ground-state effect only, describing the tendency of a given ligand to weaken the metal-ligand bond trans to itself. Herein, we will use the term "trans effect" to describe both the kinetic and the thermodynamic properties, whereas the term "trans influence" will refer only to thermodynamic properties. We will describe how these trans effects strongly impact the chemistry of the commonly encountered cyclometalated (N,C) Au(III) complexes, a class of complexes we have studied for more than a decade. We found that the outcome of reactions like alkylation, arylation, and alkynylation as well as halide metathesis are dictated by the different trans influence of the two termini of the chelating tpy ligand in (tpy)Au(OAcF)2 (tpy = 2-(p-tolyl)pyridine, OAcF = OCOCF3, tpy-C > tpy-N). There is a strong preference for high trans influence ligands to end up trans to tpy-N, whereas the lower trans influence ligands end up trans to tpy-C. Taking advantage of these preferences, tailor-made (N,C)Au(III) complexes could be prepared. For the functionalization of alkenes at (tpy)Au(OAcF)2, the higher trans effect of tpy-C would suggest that the coordination site trans to tpy-C would be kinetically more available than the one trans to tpy-N. However, due to the thermodynamic preference of having the σ-bonded ligand, resulting from the nucleophilic addition to alkenes, trans to tpy-N, functionalization of alkenes was only observed trans to tpy-N. However, for a catalytic process, the reaction should happen trans to tpy-C, as was observed for the trifluoroacetoxylation of acetylene. When functionalizing acetylene in the coordination site trans to tpy-N, protolytic cleavage of the Au-C(vinyl) bond to release the product did not occur at all, whereas trans to tpy-C protolytic cleavage of the Au-C(vinyl) bond occurred readily, in agreement with the higher trans influence of tpy-C over tpy-N. The large impact of the trans effects in Au(III) complexes is finally exemplified with the synthesis of [(tpy)Au(π-allyl)]+[NTf2]-, which resulted in a highly asymmetric π + σ bonding of the allyl moiety. Here, the bonding is such that the most thermodynamically favorable situation is achieved, with the carbon trans to tpy-N bonded in a σ-fashion and the π-allyl double bond being coordinated trans to tpy-C.

Synthesis and Biological Activity of Silicon-Containing Ethylsulfonylpyridine Insecticides

Ethylsulfonylpyridines are a novel chemical class of insecticides with excellent broad-spectrum activity and an unprecedented mode of action. With the objective of discovering novel ethylsulfonylpyridines with a broader spectrum, wider range of uses, and/or improved properties, we have started a research program aimed at introducing silicon motifs and studying their biological effects. We designed a series of Oxazosulfyl analogues where the hydrogen atom at the 5-position of the pyridyl moiety is replaced by a trialkylsilyl group and prepared these compounds applying denovo synthetic methodology. Our novel ethylsulfonylpyridines exhibit excellent insecticidal activities. The best compound, A18, resulting from our research exhibited an LC50 value of 0.30 mg/L against Plutella xylostella and reached the activity level of the commercial standard Oxazosulfyl. Our findings confirmed our working hypothesis that at the 5-position of the pyridyl moiety larger groups with different hydrophobic, electronic, and steric properties are tolerated.

Gold-catalyzed alkenylation and arylation of phosphorothioates

Reported herein is the ligand-enabled gold-catalyzed alkenylation and arylation of phosphorothioates using alkenyl and aryl iodides. Mechanistic studies revealed a crucial role of the in situ generated Ag-sulfur complex, which undergoes a facile transmetalation with the Au(iii) intermediate, thereby leading to the successful realization of the present reaction. Moreover, for the first time, the alkenylation of phosphoroselenoates under gold redox catalysis has been presented.

Gold-Catalyzed C-O Cross-Coupling Reactions of Aryl Iodides with Silver Carboxylates

We have developed a C-O cross-coupling reaction of (hetero)aryl iodides with silver carboxylates via a AuI/AuIII catalytic cycle. The transformation featured exclusive chemoselectivity and moisture/air insensitivity. Aromatic and aliphatic (including primary, secondary, and tertiary) silver carboxylates are all suitable substrates. Moreover, this protocol worked well intermolecularly and intramolecularly. Most importantly, good yields were obtained regardless of the substrates' electronic effect and steric hindrance.

Recent advances and progress in biotemplate catalysts for electrochemical energy storage and conversion

Complex structures and morphologies in nature endow materials with unexpected properties and extraordinary functions. Biotemplating is an emerging strategy for replicating nature structures to obtain materials with unique morphologies and improved properties. Recently, efforts have been made to use bio-inspired species as a template for producing morphology-controllable catalysts. Fundamental information, along with recent advances in biotemplate metal-based catalysts are presented in this review through discussions of various structures and biotemplates employed for catalyst preparation. This review also outlines the recent progress on preparation routes of biotemplate catalysts and discusses how the properties and structures of these templates play a crucial role in the final performance of metal-based catalysts. Additionally, the application of bio-based metal and metal oxide catalysts is highlighted for various key energy and environmental technologies, including photocatalysis, fuel cells, and lithium batteries. Biotemplate metal-based catalysts display high efficiency in several energy and environmental systems. Note that this review provides guidance for further research in this direction.

(P,C)-cyclometalated complexes derived from naphthyl phosphines: versatile and powerful tools in organometallic chemistry

The chemistry of (P,C)-cyclometalated complexes derived from naphthyl phosphines [^Np(P,C)M] is presented and analysed in this review. The three main synthetic approaches, namely P-chelation assisted C-H activation, oxidative addition and transmetalation, are described and compared. If a naphthyl framework inherently predisposes a phosphorus atom and transition metal to interact, a rigid metallacycle may induce some strain and distortion, as apparent from the survey of the single-crystal X-ray diffraction structures deposited in the Cambridge Structural Database (77 entries with metals from groups 7 to 11). Generally, the ^Np(P,C)-cyclometalation imparts high thermal and chemical robustness to the complexes, and a variety of stoichiometric reactions have been reported. In most cases, the metalacyclic structure is retained, but protodecyclometalation and ring-expansion have been sparingly observed. [^Np(P,C)M] complexes have also proved to be competent and actually competitive catalysts in several transformations, and they act as key intermediates in some others. In addition, interesting phosphorescence properties have been occasionally pointed out.

L-Shaped Heterobidentate Imidazo[1,5-a]pyridin-3-ylidene (N,C)-Ligands for Oxidant-Free AuI /AuIII Catalysis

In the last decade, major advances have been made in homogeneous gold catalysis. However, Au^I /Au^III catalytic cycle remains much less explored due to the reluctance of Au^I to undergo oxidative addition and the stability of the Au^III intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N-heterocyclic carbene) ligands through the development of a new class of L-shaped heterobidentate ImPy (ImPy=imidazo[1,5-a]pyridin-3-ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ-donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for Au^I /Au^III redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application.

Facile synthesis of Pt/polyoxometalate/hollow carbon sphere tri-component nanoparticles via a "double gain strategy" for high-performance electrochemical sensing of adrenaline

In this study, we designed and successfully synthesized the Pt/polyoxometalate/hollow carbon sphere (Pt/POM/HCS) tri-component nanoparticles (NPs) by a pollution-free, efficient, and convenient method. HCSs with outstanding chemical stability and conductivity are self-generated by acid etching and calcination of silica spheres synthesized by a hard template method. HCSs have a hollow internal structure that provides specific three-dimensional storage space, and can increase the surface area. The mesoporous system is beneficial to providing numerous mass transfer passageways and immobilizing NPs. In addition, we introduced a "double gain strategy", by taking advantage of POMs as reducing and bridging agents, to achieve the loading of ultrafine Pt NPs on the surface of HCSs. Pt NPs have excellent stability and unique electrocatalytic activity. As a result of the synergistic effect of HCSs and ultrafine Pt NPs, the electrochemical sensing of adrenaline exhibits high-performance catalytic activity, sensitivity, suitable linearity range (0.16 μM-1.195 mM), and low limit of detection (57.5 nM, S/N = 3), excellent stability, and reproducibility. The developed platform is a sensitive and effective adrenaline electrical sensing platform with broad practical application prospects.

Heart of gold: enabling ligands for oxidative addition of haloorganics in Au(I)/Au(III) catalysed cross-coupling reactions

The field of Au-catalysis has been an area rich with new discoveries due to the unique properties of the lustrous element. In the past decade, developments in Au(I)/Au(III) cross-coupling methodology have been made possible with the use of external oxidants that facilitate the challenging oxidation of Au(I) to Au(III) in a stable and catalytically competent fashion. Until recently, Au-chemistry was not known to undergo catalytic transformations that feature oxidative addition of haloarenes like those that were made famous by transition metals such as Pd and Ni. The discovery that ligand modification could facilitate the oxidative addition of Au(I) with haloorganics to provide Au(III) intermediates that are competent in other areas of catalysis (i.e. Lewis acid catalysis) has revolutionized this field and has led to the invention of new cross-coupling methodology. The recent advances at the leading edge in the emerging field of Au(I)/Au(III) catalysis under redox-neutral conditions are highlighted.

Enantioselective Au(I)/Au(III) Redox Catalysis Enabled by Chiral (P,N)-Ligands

Presented herein is the first report of enantioselective Au(I)/Au(III) redox catalysis, enabled by a newly designed hemilabile chiral (P,N)-ligand (ChetPhos). The potential of this concept has been demonstrated by the development of enantioselective 1,2-oxyarylation and 1,2-aminoarylation of alkenes which provided direct access to the medicinally relevant 3-oxy- and 3-aminochromans (up to 88% yield and 99% ee). DFT studies were carried out to unravel the enantiodetermining step, which revealed that the stronger trans influence of phosphorus allows selective positioning of the substrate in the C₂-symmetric chiral environment present around nitrogen, imparting a high level of enantioselectivity.

Density Functional Theory Study of Gold-Catalyzed 1,2-Diarylation of Alkenes: π-Activation versus Migratory Insertion Mechanisms

Density functional theory calculations are carried out to better understand the first gold-catalyzed 1,2-diarylation reactions of alkenes reported in the recent literature. The calculations on two representative reactions, aryl alkene/aryl iodide coupling pair (the aryl-I bond is located outside the aryl alkene) versus iodoaryl alkene/indole coupling pair (the aryl-I bond is located in the aryl alkene), confirm that the reaction involves a π-activation mechanism rather than the general migratory insertion mechanism in previously known metal catalysis by Pd, Ni, and Cu complexes. Theoretical results rationalize the regioselectivity of the reactions controlled by the aryl-I bond position (intermolecular or intramolecular) and also the ligand and substituent effects on the reactivity.

Hemilabile MIC^N ligands allow oxidant-free Au(I)/Au(III) arylation-lactonization of γ-alkenoic acids

Oxidant-free Au-catalyzed reactions are emerging as a new synthetic tool for innovative organic transformations. Oxidant-free Au-catalyzed reactions are emerging as a new synthetic tool for innovative organic transformations. Still, a deeper mechanistic understanding is needed for a rational design of these processes. Here we describe the synthesis of two Au(i) complexes bearing bidentated hemilabile MIC^N ligands, [Au^I(MIC^N)Cl], and their ability to stabilize square-planar Au(iii) species (MIC = mesoionic carbene). The presence of the hemilabile N-ligand contributed to stabilize the ensuing Au(iii) species acting as a five-membered ring chelate upon its coordination to the metal center. The Au(iii) complexes can be obtained either by using external oxidants or, alternatively, by means of feasible oxidative addition with strained biphenylene C_sp²–C_sp² bonds as well as with aryl iodides. Based on the fundamental knowledge gained on the redox properties on these Au(i)/Au(iii) systems, we successfully develop a novel Au(i)-catalytic procedure for the synthesis of γ-substituted γ-butyrolactones through the arylation-lactonization reaction of the corresponding γ-alkenoic acid. The oxidative addition of the aryl iodide, which in turn is allowed by the hemilabile nature of the MIC^N ligand, is an essential step for this transformation.

A novel hemilabile MIC^N ligand-based Au(i)-catalytic procedure for the synthesis of γ-substituted γ-butyrolactones through the arylation-lactonization reaction of the corresponding γ-alkenoic acid is presented.